If you're getting that msg, I think you're using u-verse?My understanding was u-verse has a 250G paper cap, but the meter was never working for u-verse customers (something to do with problems separating tv & internet traffic?). So it was never really enforced.

I am a U-Verse Business user and I agreed to the same 250gig cap when signing up- the TOS was the same for residential as business on the web portal login/creating a userid. Although when I login to the portal itself to check usage, it tells me I'm on unlimited usage.

The cap would need to be discussed with AT&T and see what they tell you as they decide if you are subject to them or not. Unless the portal tells you otherwise.